Parylene is a generic term applied to a class of poly-p-xylylenes which are derived from a dimer of the structure: ##STR1##
Parylene is an inert, transparent, conformal coating which has excellent barrier properties and can be used at relatively high temperatures. Due to its ability to provide thin films and conform to substrates of varied geometric shapes, it is ideally suited for use as a conformal coating in a wide variety of fields, particularly in the electronics industry.
However, the dimer itself, from which the parylene coating is prepared, is made via the pyrolysis of p-xylene or Hofmann elimination reaction of p-methylbenzyltrimethylammonium hydroxide, and and is usually obtained in relatively low yields. Accordingly, the overall process for the application of parylene as a conformal coating is expensive and severely restricts its application where it might otherwise be used.
The preparation of p-xylylene polymers by various routes has been reported in the patent literature. For example, U.S. Pat. No. 2,719,131 which issued in Sept. 27, 1955 to E.I. DuPont de Nemours and Company disclosed a process for preparing poly-p-xylene wherein the vapors of p-xylene were pyrolyzed in the presence of chlorine gas.
Also in British Patent No. 650,947 which was granted Mar. 7, 1951, polymer formation was detected on the walls of a cooling chamber after p-xylene was vaporized and pyrolyzed.
In U.S. Pat. No. 3,149,175 which issued Sept. 15, 1964 a process was reported for the preparation of di-para-xylylenes in yields of 10 percent and higher. The process involved pyrolyzing a mixture of steam and p-xylene at a temperature between about 800.degree. C. and 1000.degree. C. to generate a free radical and condensing the reactive diradical in a fluid medium.
More recently, U.S. Pat. No. 4,532,369 issued on July 30, 1985 to Hartmut Hartner of the Federal Republic of Germany and discloses and claims a process for the preparation of 2,2-p-cyclophane from p-methylbenzyltrimethylammonium hydroxide. It is indicated in the patent that known processes which existed prior to the invention disclosed gave only low yields or the starting materials were not readily accessible. By contacting aqueous p-methylbenzyltrimethylammonium hydroxide with.sodium or potassium hydroxide in the presence of dimethylsulfoxide (DMSO) the patentee indicated that yields as high as 70 percent were obtained. It was also indicated at column 1, lines 55-58, that the resulting high yields were surprising since the addition of other comparable aprotic solvents such as dimethylformamide, N-methyl-pyrollidone or sulfolane had no effect.
It is therefore an object of this invention to provide an improved process for the preparation of the dimer used in the synthesis of parylene. Another object of the invention is to provide a process for the preparation of 2,2-paracyclophane. A still further object of this invention is to provide a process for the preparation of the dimer in relative high yields as opposed to the methods disclosed in the literature to date. Another object of the invention is to provide a process which is simple and efficient and hence is effective in reducing the overall cost in the preparation of 2,2-par-cyclophane and various substituted derivatives thereof. It is also an object of this invention to provide a process which is conducted in the presence of a cosolvent and certain reaction promoters. A further object of this invention is to provide a process for the preparation of the dimer which provides high yields of the desired dimer by utilizing in combination a cosolvent and a select class of reaction promoters. A still further object of the present invention is to provide a novel process for the preparation of the dimer, 2,2-paracyclophane, more efficiently and in greater yields than heretofore. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.